I don't know if you've ever seen it, but I've definitely seen people leave comments on peoples' fic that say things like, "OMG, the way Bob jumped Pete was so hot! You should write a kidnapping fic where Bob has to rescue Pete and it's all so UST-y because he's remembering [blahblahblah]" and it's pretty obviously not about what the writer has written; it's about what the reader wants. And I think there should be boundaries around that--people should recognize that sometimes, that is your greasy id-issues. They're the kind of things that are actually subject to boundaries, even if it's just recognizing that your lustful ideas and the author's fic are not of the same part; they're separate.